1. Field of the Invention
The present invention relates to a pulse wave measuring apparatus, which is attached to a human body for measuring a pulse wave, and a method for measuring a pulse wave thereof.
2. Description of the Related Art
As an apparatus attached to the human body, such as the wrist, to measure biometric information of a user, an optical pulse wave measuring apparatus is known for measuring a pulse wave.
A pulse wave is generated when blood pressure changes. The pressure change is caused by contraction of the heart, and propagates to peripheral blood vessels. The pulse wave is caused mainly by a change of a flow rate of arterial blood flow. Since an amount of hemoglobin in the blood at a measurement location also changes similar to the flow rate of the arterial blood, it is possible to measure the pulse wave by knowing the amount of hemoglobin at a measurement location.
As one of the methods for measuring the pulse wave, a photoelectric pulse wave method is known. Light absorptivity of the hemoglobin changes depending on the wavelength of light. In the photoelectric pulse wave method, light in a wavelength band covering visible light to near infrared light, which is absorbed by the hemoglobin, is irradiated from a light-emitting element such as a light-emitting diode (LED), and the pulse wave can be known by detecting intensity variations of reflected light or transmitted light, which follows a change of blood flow, with a light-receiving element such as a photodiode.
The blood flow change in an arteriole near the skin can be known by the photoelectric pulse wave measurement method, depending on the wavelength band. Therefore, it is appropriate to conduct the measurement at a location such as the finger; the palm and the earlobe, where a large number of arterioles exist (JP-A 2003-169780).
An apparatus for measuring the pulse wave without burdening the daily life of the user is known (refer to JP-A 2005-279113 (KOKAI)). The apparatus body is put on the wrist like a wrist watch. Only a portion of a pulse wave sensor is removed from the body by a cable and wound around the finger. The pulse wave measurement is taken on a portion of a user's palm. In addition, a pulse wave interval is detected and used to learn some of the physical condition of the user. This invention enhances accuracy of a temporal resolution while sampling the pulse wave at a low speed in order to decrease power consumption. However, in this method, it is necessary to hold all data in a predetermined section of the body of the apparatus since a threshold value for use in detecting the pulse wave is calculated from a maximum and minimum value stored in the section. In order to provide a pulse wave measuring apparatus that does not put a burden on the user, it is necessary to use a microprocessor (MPU) that is compact and energy efficient. However, such a MPU has a restricted capacity RAM. Specifically, a large amount of RAM is used to detect the pulse wave interval. Thus, the useable memory capacity for other functions is decreased, which increases the possibility and decreases a degree of freedom in developing an application.
As described above, heretofore, it has been difficult to calculate a highly accurate pulse wave interval while reserving the memory necessary for pulse wave interval detection processing.